FIELD OF INVENTION
The present invention relates to a process for reducing the shock sensitivity of RDX by crystallization technique using conventional RDX prepared by nitric acid process.
BACKGROUND
In recent years, much interest has been devoted to RDX in an another form, namely, reduced sensitivity RDX (RSRDX) or insensitive RDX [IRDX), which when incorporated in cast cured plastic bonded explosive (PBX-109: RDX-64%, Al-20% & Binder-16%), can confer reduced shock sensitivity as measured through gap test [1]. Reduced sensitivity RDX is reported to possess improved crystal density, less crystal defects with smooth surface morphology. These features impart reduced vulnerability towards shock initiation. RDX is unique in nature and soluble in a wide spectrum of organic solvents. Hence, it is easy to crystallize in desired morphology. Some of the literature reports highlight the unquestionable advantage of RSRDX over conventional RDX in PBX formulation in terms of reduction in the shock sensitivity [2].
PBX formulation based on RSRDX exhibits high insensitivity towards shock initiation and requires more shock pressure to detonate the formulation. The preparation of RSRDX has been mastered in the last decade by western countries and kept under classified category. In view of its great importance, NATO countries formed a collaboration programme namely, Round Robin RSRDX (R4) for pursuing research, sharing know-how as well as the evaluation results [3]. RSRDX has been produced by many western manufacturers and R&D laboratories but the preparation method is not disclosed as on today in open literature.
I-RDX was first realized by SNPE, France [2,3] in 1990's. Later many manufacturers prepared RSRDX with special features such as high quality crystals, high density and cast PBX formulation based on RSRDX [4-7]. Reduced sensitivity RDX (RS-RDX) offers the potential to reduce the vulnerability of munitions [8-14]. This invention proposes a process for making reduced shock sensitivity RDX [RSS-RDX) from RDX prepared by nitric acid process. The reduced sensitivity RDX prepared by this invention is termed as reduced shock sensitivity RDX [RSS-RDX) and the same is used throughout the invention. The process developed by HEMRL is a proprietary crystallization technique not disclosed anywhere. The recovery of RSS-RDX from the process is about 75±5%.
The main disadvantage of conventional RDX is its high sensitivity towards shock wave initiation.
Another disadvantage of conventional RDX is that there is no uniform crystal morphology.
Yet another disadvantage of conventional RDX is that crystals possess defects which lead to hot spot generation and hence ammunitions are susceptible for accidental initiation.
Yet another disadvantage is that the crystal density of RDX is less than 1.795 g/cm3. To achieve reduced shock sensitivity RDX (RSS-RDX), selection of solvent system for crystallization is crucial. Based on the continued efforts in crystallization, a specific crystallizing medium is identified and desired morphology was achieved with improved crystal density.
SUMMARY
The main object of the present invention is to provide a process for reducing shock sensitivity of RDX to produce reduced shock sensitivity RDX (RSS-RDX) using RDX prepared by nitric acid process. This process offers particle range of varying sizes ranging from coarser [400-150 µm) to finer range of [40-90 µm) by adopting suitable cooling technique.
Another important objective of the present invention is to prepare RSS-RDX at 1 kg/batch level and the process can be scaled up to higher batch size.
Yet another object is to offer RSS-RDX of varying particle sizes ranging from coarser to fine varieties by adopting suitable cooling technique.
Yet another object is to obtain RSS-RDX of uniform, high density and defect free crystals.
Yet another object is to obtain RSS-RDX of yield of 70 %.
Yet another objective is to develop a method to differentiate RSS-RDX from any other conventional RDX using anhydrous zinc bromide solution, which can act as a quality assurance tool for RSS-RDX.
Yet another object is to process RSS-RDX in PBX formulation and determine the shock sensitivity against conventional RDX based formulations.
Accordingly the subject matter defined herein is directed to a crystallization process used for reducing the shock sensitivity of RDX comprising the steps of:
i. dissolving the RDX in a solvent selected from the group comprising of
acetone, acetonitrile, gamma-butyrolactone, ethtlacetate,
tetrahydrofuran; ii. adding co-solvent selected from the group comprising of di
meyhylformamide, di methylsulfoxide, formamide, diethyl carbonate,
N-methyl pyroUidinone to the solution obtained in (i); iii. raising and maintaining the temperature in the range from 55 to 80°C; iv. addition of non solvent selected from the group comprising of water,
methanol, iso-propanol, n-butanol, tert-butanol to the solution
obtained in (iii); and v. cooling the solution obtained in (iv) to obtain reduced sensitivity RDX
crystals.
The process has a yield of 70%, Further, this invention offers a method of identifying and distinguishing RSS-RDX from conventional RDX by a sedimentation method using aqueous zinc bromide solution of density 1.795 g/cm3.
The subject matter disclosed herein is also directed to process to prepare and evaluate, a PBX formulation comprising RSS-RDX/Aluminium/HTPB binder in 64/20/16 proportion for shock insensitivity.
The subject matter disclosed herein is also directed to the method of processing RSS-RDX in PBX formulation [RSS-RDX: 64%, A1: 20% & Binder: 16%] and subjected to large scale gap test to obtain the minimum shock pressure required for initiation of the charge.
BRIEF DESCRIPTION OF FIGURES
These and other features, aspects, and advantages of the present invention will
become better understood when the following detailed description is read with reference to
the accompanying drawings in which like characters represent like parts throughout the
drawings, wherein:
Fig. 1 illustrates a SEM picture of Conventional RDX (Fig. 1(a)) and a SEM picture of Crystal morphology of RSS-RDX (Fig. 1 (b)).
Fig. 2 illustrates results of a test for Identiflcation of RSS-RDX. Fig 2(a) shows test results of (a) Conventional RDX which floats completely on Zinc bromide solution of density 1.795(1) and Fig 2(b) shows test results of (b) RSS-RDX which is sedimented.
DESCRIPTION OF THE INVENTION
The present invention provides a process of reducing the shock sensitivity of RDX comprising the steps of:
i. dissolving the RDX in a solvent selected from the group comprising of acetone,
acetonitrile, gamma-butyrolactone, ethtiacetate, tetrahydrofuran; ii. adding co-solvent selected from the group comprising of di meyhylformamide, di
methylsulfoxide, formamide, diethyl carbonate, N-methyl pyrollidinone to the
solution obtained in [i]; iii. raising and maintaining the temperature in the range from 55 to 80°C; iv. addition of non solvent selected from the group comprising of water, methanol, iso-
propanol, n-butanol, tert-butanol to the solution obtained in (iii]; and V. cooling the solution obtained in [iv) to obtain reduced sensitivity RDX crystals.
The shock sensitivity of RDX is reduced by a crystallization technique. The details of the solvent mixture are given in Table 1. In this method of preparation, conventional RDX is dissolved in a mixture of solvent [Row 1 of Table 1) / co-solvent [Row 2 of Table 1] at 75±5°C under stirring. To this non-solvent [Row 3 of Table 1) is added drop wise under stirring. RSS-RDX is obtained by slow cooling to ambient temperature over a period of 2 h. This method offers RSS-RDX of varying particle sizes for insensitive munitions.
Table 1: Details of organic solvent mixture used for preparation of RSS-RDX
(Table Removed)
In specific embodiment of the present invention the process further comprising the step of filtering under vacuum, washing and drying of RDX crystals.
In specific embodiment of the present invention the process step [i) and [ii) are carried out simultaneously under continuous stirring.

In specific embodiment of the present invention the non-solvent is added drop wise over a period of 5 min to 2 hrs.
In specific embodiment of the present invention the the non-solvent is added under stirring at a speed in the range of from 150 to 200rpm.
In specific embodiment of the present invention the process the amount of solvent and co-solvent used are in the ratio in the range of from 3:4 to 5:4.
In specific embodiment of the present invention the amount of solvent and co-solvent used is in the ration of 1:1.
In specific embodiment of the present invention the ratio of amount of solvent to co-solvent to non-solvent is in the ratio of 1:1:4.
In specific embodiment of the present invention the ratio of amount of solvent, co-solvent and non-solvent to RDX is 1:4 (w/v].
In specific embodiment of the present invention the the RDX crystals are washed with 25 % to 100 % aqueous solution of non-solvents selected from the group comprising of water, methanol, iso-propanol, n-butanol, tert-butanol.
In specific embodiment of the present invention the RDX crystals are dried at a temperature in the range of from 30-50 °C.
In another aspect of the present invention the RDX with reduced shock sensitivity is used in the preparation of a PBX formulation comprising: RDX with reduced shock sensitivity; HTPB, di octyl adipate; lecithin; butyl stearate; dibutyl tin dilaurate, aluminium, and isophorone diisocyannate.
In specific embodiment of the present invention Shock Sensitivity RDX is reduced by:
(a) Placing a three necked round bottom flask of 5 litre capacity with stirring assembly
over a water bath with provision of hot / cold water circulation
(b) Assembling a three solvent reservoir (solvent, co-solvent and non-solvent) with the flask through a three-way cock
(c) Charging the flask with appropriate solvent from Row 1 of Table 1 under stirring [150±50 RPM)
(d) Addition of RDX portion wise into the flask
(e) Charging the flask with appropriate co-solvent from Row 2 of Table 1
(f) Filling and circulation of hot water into the water bath for heating the contents of the flask
(g) Raising and maintaining the temperature of the solvent mixture to 55-75°C to dissolve the RDX under stirring (150±50 RPM)
(h) Addition of suitable non-solvent from Row 3 of Table 1 into the solution drop-wise for a period of 10 minutes by maintaining the temperature at 55-75°C under stirring
(i) Cooling the solution by circulating ice cooled water under stirring (150±50 RPM)
(j) Filtration of RSS-RDX under vacuum
(k) Washing the RSS-RDX crystals with aqueous solution of non-solvent (water 50% & non-solvent 50%) to remove solvent impurities.
(1) Washing the RSS-RDX crystals with aqueous solution of non-solvent (water 75% & non-solvent 25%).
(m) Drying of RSS-RDX in the temperature range of 40-50°C for about 2 h.
In specific embodiment of the present invention the RSS-RDX are identified by using Zinc Bromide of Density 1.795 by:
a) Weighing of calculated quantity of anhydrous zinc bromide and taken in a 1 liter standard measuring flask (SMF).
b) Addition of double distilled water into the SMF to make up to the level.
c) Ensuring the solution density by a density balance and fixing the density at 1.795 g/cm3 by adding either distilled water or zinc bromide solid.
d) Addition of 20 ml of zinc bromide solution into two separating funnels (marked as RSS-RDX and RDX)
e) Placing of about 5 g of each RSS-RDX and RDX samples into the appropriate separating funnels and mixing contents well
f) Observation of the flasks for sedimentation /floatation pattern
In specific embodiment of the present invention RDX with reduced shock sensitivity is used in the preparation of a PBX formulation as follows:
a) Mixing of weighed quantities of hydroxy! terminated poly butadiene (HTPB), dioctyl adipate (DOA) along with lecithin, butyl stearate and dibutyl tin dilaurate [DBTL) to the bowl of the mixer for 5 minutes without vacuum and about 20 minutes under vacuum
b) Addition of 6% dioctyl phthalate (DOP) coated fine grade RSS-RDX (~ 125 µm] into the bowl of the mixer
c) Mixing the ingredients without vacuum for about 10 minutes and under vacuum for about 30 minutes
d) Addition of RSS-RDX (~ 250 µm) into the mixing bowl and mixed for about 10 minutes without vacuum and about 30 minutes under vacuum
e) Addition of aluminium powder (~ 25 µm) and mix for about 10 minutes without vacuum and about 30 minutes under vacuum
f) Addition of weighed quantity of the curative isophoron diisocyanate [IPDI] and continue mixing without applying vacuum for about 5 minutes.
g) Continuing mixing for another 20 minutes under vacuum.
h) Filling the composition in the shock sensitivity tubes under vacuum i) Curing the tubes at 60 ± 5°C for about 7 days in water jacketed oven
WORKING EXAMPLE
1. Preparation of Reduced Shock Sensitivity RDX (RSS-RDX)
A three necked glass round bottom flask of capacity 20 liter with a mechanical stirrer assembly was placed over a water bath (50 lit capacity). This set-up was connected to three reservoirs containing solvent, co-solvent and non-solvent through a three way cock. Thermometer was inserted to measure the temperature of the solvent mixture. About 10.0 lit of solvent (5.0 lit of acetonitrile and 5.0 lit of diethyl carbonate) were charged into the flask. About 3.0 kg of RDX was charged portion wise under stirring (150±50 RPM). The temperature of the set-up was raised to 55-75°C and maintained until all RDX was dissolved. Methanol (2.0 lit) was added drop-wise into the solvent over a period of 10 minutes under stirring. The contents of the flask were allowed to cool to ambient temperature with ice. RSS-RDX precipitated was filtered under vacuum and washed with 2
lit of 50% aqueous methanol (1.0 lit water + 1.0 lit methanol) to remove traces of solvent RSS-RDX crystals were washed again with 2 liters of 25% aqueous methanol [1.5 lit water + 0.5 lit methanol) and subsequently with plenty of distilled water. The obtained RSS-RDX was dried at 45±5°C [2.1 kg, yield: 70 %).
Table 2. Characteristic properties of RDX and RSS-RDXt
The physico-chemical characteristics of RDX and RSS-RDX are presented in the Table 2. The results reveal that RSS-RDX resembles RDX in all characteristics but differs in morphology.
2. Identification of RSS-RDX
About 280 g of anhydrous zinc bromide (1.243 mol) was weighed accurately and placed in 1 liter standard measuring flask Distilled water was added to the standard measuring flask (SMF) and made up to the 1 liter mark The density measurement balance was initially kept ready for density measurement of the solution. About 100 ml of the above solution was taken in a beaker and placed in the pan of the balance. Glass plummet was immersed in the liquid and reading was noted, which is a direct measure of the density of the zinc bromide solution. The solution density was adjusted to 1.795 g/cm3 by adding either distilled water or zinc bromide solid.
About 5 g each of RSS-RDX and RDX were weighed and taken in 250 ml separating funnels, suitably marked. About 20 ml of above zinc bromide solution was added into the above flasks, shaken well and kept aside for monitoring the sedimentation pattern. RSS-RDX crystals completely sedimented within an hour whereas RDX crystals were floating over the liquid surface (Fig. 2). This test clearly distinguishes RSS-RDX and conventional RDX.
3. Processing in PBX formulation (Table 3)
PBX of batch size 3510.78 g was processed in a vertical planetary mixer. Initially, the binder ingredients HTPB (259.7 g), DOA (187.6 g), butyl stearate (44.1 g), lecithin (10.5 g) and DBTL (0.28 g) were added to the bowl of the mixer (Table 3). The binder ingredients were mixed for 5 minutes without vacuum and 20 minutes under vacuum. About 0.735 kg fine grade RDX (113 µm) was coated with DOP (0.047 kg) separately and then introduced into the bowl of the mixer. The ingredients were mixed without vacuum for 10 minutes and under vacuum for 30 minutes. Mixing was stopped and 1.505 kg of RDX (207 µm) was added to the mixing bowl and mixed for 10 minutes without vacuum and 30 minutes under vacuum.
Table 3: Composition: RDX / A1 / HTPB (64/20/16)
Mix level = 3.51078 kg
(Table Removed)
Finally, 0.7 kg aluminium powder (15 µm) was added and mixed for 10 minutes without vacuum and 30 minutes under vacuum. Mixing was then stopped and IPDI (0.0217 kg] was added and mixing continued without applying vacuum for 5 minutes. Further mixing for another 20 minutes under vacuum was done. Mixing was stopped, contents were removed and filled in the mould under vacuum and cured at 60 ± 5°C for 7 days in water-jacketed oven. Similarly, RDX based charge was formulated for comparison purpose.
4. Determinations of shock sensitivity of RSS-RDX based PBX formulation
RSS-RDX & RDX based PBX charges were subjected to large scale gap test by following the established procedure [15]. RSS-RDX based PBX formulation was shock insensitive upto 48.4 kbar shock pressure whereas RDX based charge was upto 24.7 kbar. This reveals that PBX formulation containing RSS-RDX is doubly insensitive to shock pressure than RDX based formulations, which ensures the safety features of ammunition by 96 %.
It is to be understood that the process of the present invention is susceptible to modifications, changes and adaptations by those skilled in the art Such modifications, changes and adaptations are intended to be within the scope of the further set forth under the following claims.
REFERENCES
1. I.J. Lochert, M.D. Franson, and B.L. Hamshere, Reduced Sensitivity RDX Part I: Literature Review and DSTO Evaluation. DSTO-TR-1447, (2003), DSTO.
2. A. Freche, C.Spyckerelle, and S. Lecume, SNPE Insensitive Nitramines, Insensitive Munitions and Energetic Materials Technology Symposium (2003), Orlando, Florida.
3. Lecume, S., Chabin, P., and Brunet, P. Two RDX Qualities for PBXN-109 Formulation Sensitivity Comparison, 2001 Insensitive Munitions and Energetic Materials Symposium [2001), Bordeaux.
4. R. M. Doherty, G. W. Lawrence, H. W. Sandusky and R. H. Granholm, "Witness Dents in Shock Reactivity and Sensitivity Studies," Proceedings of 23rd JANNAF Propulsion Systems Hazards Subcommittee Meeting, JSC CD-46, Dec 2006.
5. R. M. Doherty, L. A. Nock and D. S. Watt, "Reduced Sensitivity RDX Round Robin Programme - Update," Proceedings of the 37th International Annual Conference of ICT on Energetic Materials, 27 - 30 June 2006, Karlsruhe, Germany.
6. Doherty, R., Sandusky, H., Lochert, I., and Hamshere, B., "Further Studies Examining the Shock Sensitivity of RS-RDX," Insensitive Energetic Materials Symposium -Particles, Crystals, Composites, Fraunhofer ICT, Pfinztal, Germany, 6-7 Mar 2007, pp. 129-141.
7. Sandusky, H. W., Felts, J. E., Granholm, R. H., and Doherty, R M., "Gap Test Phenomena - PBXN-109 Data For Different RDX Fills and Arrangements," 24th JANNAF Propulsion Systems Hazards Subcommittee Meeting, JSC CD-53, Chemical Propulsion Information Analysis Center, May 2008.
8. Lochert, I.J., Dexter, R.M., and Hamshere, B.L. Evaluation of Australian RDX in PBXN-109. DSTO-TN-0440, (2002), DSTO.
9. Lochert, I.J., Franson, M.D., and Hamshere, B.L. Assessment of Australian Insensitive RDX, Insensitive Munitions and Energetic Materials Technology Symposium (2003), Orlando, Florida.
10. Freche, A., Aviles, J., Donnio, L., and Spyckerelle, C. Insensitive RDX (I-RDX), Insensitive Munitions and Energetic Materials Symposium - Technology Implementation in the 21st Century (2000), San Antonio, Texas.
11. Franson, M.D., Lochert, I.J., and Hamshere, B.L. DSTO Evaluation of Reduced Sensitivity RDX (RS-RDX), Parari, Sixth Australian Ordnance Symposium (2003), Canberra, Australia.
12. Scholtes, G. and van der Heijden, A. IM-Related and Laboratory Scale Testing on I-RDX and RDX-Based Explosives, Insensitive Munitions and Energetic Materials Technology Symposium (2004], San Francisco, California.
13. Spyckerelle, C, Freche, A., and Eck, G. Ageing of I-RDX and of compositions based on IRDX, Insensitive Munitions and Energetic Materials Technology Symposium (2004), San Francisco, California.
14. Kristiansen, J.D., Johansen, 0.H., Berg, A., Halvorsen, T., Gjers0e, R., Smith, K.-T., and Christensen, M. Steps towards reduced sensitivity HMX (RS-HMX): Reduced shock sensitivity in both Cast-Cured and Pressable PBX compositions., Insensitive Munitions and Energetic Materials Technology Symposium (2004), San Francisco, California.
15. Donna Price, Clairmont Jr., A. R. and Erkman, J. 0., The NOL large scale GAP test III. Compilation of unclassified data and supplementary information for interpretation of results, NOL TR 74-40, AD-780 429, 8 March 1974, Naval Ordnance Laborateory, Maryland, USA.

I/We claim:
1. A process of reducing the shock sensitivity of RDX comprising the steps of:
i. dissolving the RDX in a solvent selected from the group comprising of
acetone, acetonitrile, gamma-butyrolactone, ethtlacetate, tetrahydrofuran; ii. adding co-solvent selected from the group comprising of di
meyhylformamide, di methylsulfoxide, formamide, diethyl carbonate, N-
methyl pyrollidinone to the solution obtained in (i); iii. raising and maintaining the temperature in the range from 55 to 80°C; iv. addition of non solvent selected from the group comprising of water,
methanol, iso-propanol, n-butanol, tert-butanol to the solution obtained in
(iii); and V. cooling the solution obtained in (iv) to obtain reduced sensitivity RDX
crystals.
2. The process as claimed in claim 1, further comprising the step of filtering under vacuum, washing and drying of RDX crystals.
3. The process as claimed in claim 1, wherein step (i) and (ii) are carried out simultaneously under continuous stirring.
4. The process as claimed in claim 1, wherein the non-solvent is added drop wise over a period of 5 min to 2 hrs.
5. The process as claimed in claim 1 and 4, wherein the non-solvent is added under stirring at a speed in the range of from 150 to 200rpm.
6. The process as claimed in claim 1, wherein the amount of solvent and co-solvent used are in the ratio in the range of from 3:4 to 5:4.
7. The process as claimed in claim 1 and 6, wherein the amount of solvent and co-solvent used is in the ration of 1:1.
8. The process as claimed in claim 1, wherein the ratio of amount of solvent to co-solvent to non-solvent is in the ratio of 1:1:4.
9. The process as claimed in claim 1, wherein the ratio of amount of solvent, co-solvent and non-solvent to RDX is 1:4 (w/v).
10. The process as claimed in claim 1, wherein the RDX crystals are washed with 25 % to 100 % aqueous solution of non-solvents selected from the group comprising of water, methanol, iso-propanol, n-butanol, tert-butanol.
11. The process as claimed in claim 1, wherein the RDX crystals are dried at a
temperature in the range of from 30-50 °C.
12. The RDX with reduced shock sensitivity as produced by the process claimed in claim 1 to 11 as and when used in the preparation of a PBX formulation comprising: RDX with reduced shock sensitivity; HTPB, di octyl adipate; lecithin; butyl stearate; dibutyl tin dilaurate, aluminium, and isophorone diisocyannate.